Fastest WP theme?

GeneratePress
This theme comes with advanced customization options as well.
Astra
This theme comes with a real-time customizer so you can see your creation unfold in front of your eyes.

If you connect several plugins or even constructors for example Elementor. The speed of your "fastest theme" will slow down significantly. It is not the theme that is important, but the optimization of all the components of your site. Your host, CDN, image optimization, scripts and font optimization etc is important more than "theme speed".

Most likely in the coefficients.